1. Technical Field
The present invention relates to a high-melting point flame retardant crystal and method of preparing the same, a flame retardant-containing epoxy resin composition having excellent heat resistance and high-temperature reliability and having low moisture absorption and low water absorbency, a prepreg using the composition, and a flame-retardant laminate using the prepreg. More particularly, the present invention relates to an epoxy resin composition containing an additive such as a curing agent, the composition being prepared by dispersing flame retardant powder, having a start melting point of 280° C. or more and a melting point of 291° C. or more and having reactivity to epoxy resin, in uncured epoxy resin such that the flame retardant powder does not react with the uncured epoxy resin, a prepreg obtained using the composition, and a flame-retardant laminate obtained by thermally pressing the composition to cure the composition and simultaneously fixing a flame retardant.
2. Description of the Related Art
Epoxy resins have been used as an electrical insulating material because they have excellent electrical characteristics. Particularly, epoxy resins are used to manufacture laminates and printed substrates.
Although a printed substrate is provided with parts, such as LSI, IC and the like, connected and fixed by soldering, when conventional solder including lead is used, the environmental friendliness cannot be improved. For this reason, it is necessary to use lead-free solder having a melting point higher than conventional solder, so that a laminate also requires higher heat resistance and improved reliability such that it can cope with high-temperature soldering.
Further, in order to manufacture small-size high-performance electric and electronic appliances, for example, mobile phones, it is required to dispose wiring on a laminate at narrow intervals and to increase the density of the wiring. Therefore, it is necessary to increase the heat resistance of the laminate and decrease the linear expansion coefficient thereof even for the purpose of preventing a substrate from cracking or preventing wiring from being severed by thermal expansion at high temperature.
Further, since a high-density printed substrate is easily negatively affected by foreign matter such as moisture in the air, it is necessary to increase the long-term reliability and to keep the invasion of foreign matter to a minimum.
A printed substrate is provided with minute and complicated electric wiring, and is treated with a flame retardant in order to prevent a fire from breaking out because of a short or the like.
Conventionally, as laminates for printed substrates, laminates manufactured using an epoxy resin flame-retarded by a bromine compound have been generally used, but, recently, laminates using a phosphorus-based flame retardant have also been used. It is known that a phosphorus-based flame retardant has high thermal stability and contributes to the reduction in weight of a laminate compared to a bromine-based flame retardant.
Patent documents 2˜6 disclose epoxy resins using a phosphorus-based flame retardant.
Patent documents 2˜5 disclose methods of manufacturing a laminate using a flam-retardant epoxy resin obtained by reacting an organic phosphorus-based flame retardant with an epoxy resin. In these methods, since the solubility of the flame-retardant epoxy resin in a solvent is improved, uniform varnish can be maintained, so that the flame-retardant epoxy resin is easily impregnated in a substrate made of glass fiber or the like.
However, the solubility of the flame-retardant epoxy resin in a solvent is improved, but there is a problem that a prepreg or laminate manufactured using the flame-retardant epoxy resin is easily affected by the invasion of moisture because the organic phosphorus-based flame retardant reacting with an epoxy resin has a noncrystalline structure.
Patent document 5 discloses a flame retardant (Patent document 1) having the same molecular structure as that of the flame retardant used in the present invention. The flame retardant disclosed in patent document 5 has a start melting point of less than 280° C. and a melting point of less than 291° C., which were measured by differential thermogravimetric analysis. When a laminate is formed using the flame retardant, the flame retardant entirely reacts with an epoxy resin to lose crystallinity, so that the noncrystallized flame retardant is easily affected by the invasion of foreign matter such as moisture or the like, with the result that it is difficult to obtain heat resistance and high-temperature reliability required to cope with lead-free solder.
Further, Patent document 6 discloses a technology for improving the working efficiency by dispersing an unreacted organic phosphorus-based flame retardant in varnish.
However, even when this technology is used, since the melting point of the flame retardant is less than 265° C. at which solder resists heat, this flame retardant cannot maintain its crystalline structure to be melted at a temperature of lower than 265° C., so that its heat resistance and high-temperature reliability are not yet satisfactory.